1. Scope of Invention
This invention relates generally to the field of turbidity/nephelometry and particularly to an improvement in the stability and accuracy of the test suspension used as test samples to accurately calibrate turbidity/nephelometry measurement equipment.
2. Prior Art
The present invention represents a substantial improvement over my prior teachings as set forth in U.S. Pat. No. 4,291,980. To the extent not reproduced or derived from the '980 patent, that disclosure is incorporated herein by reference.
In testing and treating water for drinking purposes, it is necessary to test the water's Turbidity. Turbidity has a marked effect on the bacteriological quality of water, whether or not disinfection is practiced. This is so because turbidity interferes with the ability to disinfect water.
Turbidity is measured by use of a turbidimeter which includes a light source for illuminating a sample to be tested and one or more photoelectric detectors with a readout to indicate the intensity of light scattered at right angles to the path of the incident light. The greater the scatter, the greater the turbidity. In testing for turbidity, a turbidity reference suspension must be selected which is readily reproducible and which can be used to calibrate the turbidimeter. Until the advent of the '980 patent teaching, no such reference existed.
Prior attempts to measure turbidity were, in retrospect, rather crude. FIG. 1 shows the first accepted means of evaluating turbidity by filling a cylindrical container 1 with water 3 to a height 5. Candle 2 was placed at the bottom of the transparent cylindrical vessel 1. The illumination produced by the candle 2 was viewed downwardly toward the water surface 4. Simply stated, water level 5 was increased until the candle's illumination could no longer be seen at 4. The technique was proposed by Jackson and the height of the water was read in Jackson Turbidity Units (JTU). The result was a crude determination of the turbidity of water for as water became more turbid, the height 5 became smaller for a given sample.
The Jackson method can only be described as primitive at best. The candle would blacken the bottom of the transparent vessel thus interfering with the pure turbidity measurement. Furthermore, sedimentation would precipitate out of solution and would block the candle's illumination, although such sedimentation has nothing to do with turbidity.
The next advance in turbidity measurement involved the use of Formazin suspended in water as the reference. Formazin is the condensation polymer of hydrazine sulfate (NH.sub.2).sub.2 H.sub.2 SO.sub.4 and hexamethylenetetramine C.sub.6 H.sub.12 N.sub.4. Unfortunately, hydrazine compounds are extremely toxic end their use as a preparation of a turbidity standard for water represents certain disposal problems and health problems which should be avoided if possible. A further drawback to the use of Formazin as a standard is that in the 1.0 and 5.0 NTU range, a non-linear dilution of Formazin concentrate is necessary for, in such low concentrations, Formazin decomposes. Formazin is prepared via a standard condensation reaction: ##STR1## As the Formazin solution becomes more dilute, the reaction is pushed to the left, thus breaking down the compound.
A further drawback experienced through the use of Formazin is that it characteristically has a sedimentary light scatter loss of approximately 10% for four hours. This means that care must be given to thoroughly mix any Formazin suspension prior to sampling. Such a problem can be more readily appreciated by viewing FIG. 2. The turbidity of a Formazin containing standard is done by illuminating transparent cylinder 7 through its base by light source 9. The Formazin 15 suspended in water 6 causes the light to scatter and a light reading is taken at rights angles to the incidence of illumination at 8. The turbidity units are known as Formazin Turbidity Units (FTU). Again, the greater turbidity, the greater the light scatter.
Because of the problems outlined herein, a Formazin standard, although in use for many years, is not at all accurate. The sedimentary light scatter loss is significant and, as was true with regard to the Jackson turbidity test, sediment acts to block the illumination source and results in false readings of the standard. Lastly, Formazin diluted has a life expectancy of approximately one week while in a concentrated form, its life expectancy is approximately 30 days.
It was also found that in order to provide a proper material for use as a standard in turbidity measurements, the particle would have to have an extremely long shelf life and be of a size which would approximate those impurities which normally cause water to be turbid such as spores and bacteriological growth. It was also found necessary to produce a particle which, when suspended in water, would remain suspended for an extremely long period of time so that the turbidity measurement can be taken without sedimentation. All of those necessary characteristics were achieved in producing the specific copolymer of styrene and divinyl benzene of my '980 patent and the present invention. However, the teachings of the '980 patent do not provide for suspension samples at very low levels of turbidity, e.g. below 1.0 NTU and as low as 0.02 NTU.
The production of spherical beads comprised of copolymers styrene and divinylbenzene is well known. For example, see U.S. Pat. Nos. 2,366,007 and 3,463,320. However, until the advent of my '980 patent and as improved upon by the present invention, no material such as a styrenedivinylbenzene copolymer could possibly act as an acceptable standard in turbidity measurement. More specifically, the particle would be spherical in nature and have a diameter approximately in the range of 0.2 microns to 1.0 microns. On a statistical basis, this means that approximately 90% of all beads produced should fall within the range.
There are two basic types of polymerization systems; i.e., suspension and emulsion polymerizations. In suspension polymerization, which is classically an oil in water suspension wherein the oil phase is polymerized by the introduction of free radicals while the oil droplet suspension is maintained, the particles produced tend to have a diameter greater than 1.0 micron. If a classical emulsion polymerization is performed, wherein the polymer is built up from a solubilized phase of monomer, particles much smaller than 0.1 micron are produced. Thus, in order to arrive at an ideal particle size, my former '980 patent developed a unique polymerization process which is a cross between suspension and emulsion polymerization.
The national standard test for turbidity of water is described in ASTM Designation D 1889-94. The currently accepted test equipment for turbidity measurement is the Mixcro 200 BW produced by HF Scientific Corp., a Florida corporation.
Although the turbidity sample standard and method of its preparation disclosed in my '980 patent has also been used recently and has been adopted by the E.P.A as the definitive means for testing the turbidity of water, there remains serious drawbacks to that teaching and the accuracy and reproducibility of test sample suspensions especially below 1.0 NTU. The difficulty with the '980 teaching is with respect to the dilution of the sample to lower NTU units to provide a calibration sample below. The only means for dilution available under the '980 teaching was through the addition of additional Tergitol 4T, a sodium alkyl sulfate anionic pure surface active agent produced by the Union Carbide Company. This addition of 4T prevent the diluted sample from becoming unstable by agglomeration. Although it is accurately claimed in the '980 disclosure that the particles in the test sample remain stable for at least two year, the diluting addition of additional Tergitol 4T may destroy that stability and, more importantly, it becomes virtually impossible to determine the exact gravimetric analysis of the particles because of the presence of the Tergitol 4T which cannot be removed without destroying the sample.
The present invention adds a substantial improvement to the teachings of my previous '980 patent by overcoming these severe limitations of gravimetric analysis inaccuracy through the use of ammonia as a dilutant which is added in sufficient quantity to elevate the pH of the media to an alkaline level of at least about pH 8 or above.